A heat-sensitive recording sheet forms images by physical or chemical changes of materials by heat energy. A number of processes for forming images with such sheets have been studies.
An example of a heat-sensitive recording sheet utilizing physical change of materials by heat is wax type heat-sensitive recording paper which is utilized for cardiograms as described in U.S. Pat. No. 3,131,080. With respect to heat-sensitive recording sheets utilizing chemical change, various kinds of development mechanisms have been proposed as described in U.S. Pat. Nos. 2,663,654, 2,663,655 and 2,967,785. A typical example is a two-component development heat-sensitive recording sheet.
The two-component development heat-sensitive recording sheet is prepared by finely dispersing two kinds of heat-reactive compounds, adding a binder and applying the dispersion to a base so that two kinds of heat-reactive compounds are separated from each other by the binder. Recording is carried out by a color forming reaction which takes place between the reactive compounds after heat is applied and they contact each other. These two kinds of heat-reactive compounds are generally called an electron donating compound and an electron accepting compound, and there are a considerable number of combinations of them. Roughly classified, combinations which form metal compound images and combinations which form dye images are known.
These two-component development heat-sensitive recording sheets have a number of advantages, for example, (1) development is not necessary because of primary color formation, (2) the quality of the paper used is similar to conventional paper, and (3) handling of them is easy. Particularly, when using a colorless dye as the electron donating compound, there are some additional advantages such as (4) the color density is high and (5) heat-sensitive recording sheets yielding various color hues can be easily produced and their utilization value is great. Accordingly, they have been broadly used as heat-sensitive recording materials.
The heat-sensitive recording sheets having the above described excellent advantages have recently been utilized as image receiving paper for recording facsimile communication.
When the heat-sensitive recording sheets are used as recording paper for a facsimile, the construction of instruments is simplified, because the development step is not necessary. Furthermore, no material other than the recording paper need be consumed and there are many advantages with respect to maintenance. However, such materials are not desirable because during thermal recording the recording speed is low. This is believed to be caused by the fact that the thermal recording head and the heat-sensitive recording material used have inferior heat-response. Although thermal recording heads having a good heat-response have been developed in recent years, a heat-sensitive recording sheet which sufficiently satisfies the requirement has not yet been provided. Thus, it has been desired to develop such heat-sensitive recording sheets.
The present inventors have thought that an important matter for attaining the above described object is to uniformly and finely disperse the electron donating colorless dye (referred to as color former, hereinafter) and the electron accepting compound (referred to as color developer, hereinafter) in an aqueous solution. It has been found that a horizontal sand mill gives excellent results with respect to dispersing.
In a conventional process for producing heat-sensitive recording sheets, a color former and a color developer are dispersed, respectively, in an aqueous solution of a water-soluble binder by means of a ball mill and they are mixed. Thereafter the resulting dispersions are applied to a base after adding, if necessary, organic or inorganic pigments, waxes or releasing agents. Production of the dispersion of fine uniform particles in the dispersing step in this process is a primary factor for determining the heat-response. Previously, ball mills, attriters, sand mills, and three-roller mills have been used for dispersing. However, the ball mills and attriters have an inferior operation efficiency, because they are used as a batch system, and the particle size of the resulting dispersion in case of using conventional color formers and color developers is 3 .mu.m to 5 .mu.m on the average, which is not always sufficiently small. Moreover, some attriters and sand mills cannot yield uniform fine particles, though continuous operation can be carried out, and the average particle size is also limited to 3 .mu.m to 5 .mu.m or so at most. Further, the distribution of particle size is rather broad.
As a result of studying these matters, the present inventors have made the following discoveries. When using ball mills and attriters, etc., the quantity of motion of the balls or media is small. Accordingly, production of fine particles is naturally limited because only a small shearing force is applied to the dispersoid. Further, when using the prior continous attriters and sand mills, a vessel is provided in the perpendicular direction so that the dispersion medium flows (generally) towards the upper part from the lower part. Accordingly, the distribution of concentration of the dispersoid is formed in the vertical direction of the vessel based on specific gravity differences between the dispersiod and the dispersion medium resulting in a dispersion with broad distribution of particle size. Accordingly, it is difficult to easily produce fine particles. This tendency becomes great when the amount of flow of the dispersion medium is reduced in order to obtain fine particles. Further, when heat-fusible substances such as a melting point dropping agent are dispersed simultaneously with the color former and the color developer in order to improve the heat-response of the heat-sensitive recording sheet, separation of the raw materials is caused during dispersing because each has a different specific gravity. Consequently, there is the problem that the composition of the vomited finely dispersed solution or the particle size of dispersoids thereof is different from that of the crude dispersion to be introduced. In general, since the specific gravity of a melting point dropping agent is small, the composition of the vomited solution contains a melting point in a large proportion at the initial dispersion. Further, the melting point dropping agent is easily vomited. Accordingly, the dispersing time in dispersing vessel is short and the extent of dispersing is relatively small. Contrary thereto, the time for dispersing the color former and color developer is longer than the case of the melting point dropping agent. Accordingly, the crushing thereof is more proceeded and the particle sizes of the color former and color developer become small as compared with that of the melting point dropping agent.